Problem: Solve for $p$ : $15 + p = 13$
Answer: Subtract $15$ from both sides: $ \begin{eqnarray} \\ 15 + p &=& 13 \\ \\ {-15} && {-15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{15 + p} &\hphantom{=}& \hphantom{13} \\ p &=& 13 {- 15} \\ \end{eqnarray} $ Simplify. $ p = -2$